1. Field of the Invention
The present invention relates to an image reading device, and, more particularly, an image reading device having for example a linear image sensor and reading image information by relative movement of an information bearing member, such as an original document, in contact with said linear image sensor, and adapted for use in a facsimile apparatus, a copying machine, an image reader or the like.
2. Related Background Art
In the field of image reading device utilizing linear image sensor, there is already known a device for reading an original image by focusing said image onto a linear image sensor of a length of several centimeters in the main scanning direction, through a reduction optical system. However such device requires a large optical path for reduction imaging, and cannot be easily compactized since the optical system requires a large space for housing it.
On the other hand, the use of an equal-size optical system employing a linear image sensor of a length substantially equal to the width of the original image allows to significantly reduce the space required for housing the optical system, thereby compactizing the reading apparatus. Such equal-size optical system is already known to be realizable with light concentrating fibers or with a contact lens array. Also recently there has been developed an image reading device of contact type in which the original document is moved in contact with the linear image sensor without such fibers or lens array, as disclosed in the Japanese Patent Laid-open Nos. 74262/1980, 75271/1980, 45084/1981 and 122172/1981 by the present applicant.
FIG. 1 is a partially cut-off lateral cross-sectional view of a principal portion of such a conventional image reading device of contact type, provided with photosensor portions or photosensor elements 8, arranged on a transparent substrate 11 such as glass, in a direction perpendicular to the plane of drawing to constitute a linear image sensor.
Said sensor element 8 is composed, on a transparent substrate 11 such as glass, of an opaque layer 12 of an opaque material, such as metal; an insulating layer 13 of an electrically insulting material; a semiconductor layer 14 of a photoconductive semiconductor material such as hydrogenated amorphous silicon (a-Si:H) or CdS.Se; a semiconductor layer 15 doped with semiconductive impurities for forming ohmic contacts; and a pair of main electrodes 16, 17, defining a light-receiving window 18 therebetween.
In such a structure, incident light L entering the transparent substrate 11 through an entrance window 19 formed thereon (the sensor 8 is protected from said incident light by the opaque layer 12) illuminates an original document P, and the reflected light is received by the light-receiving window 18 of the sensor 8, whereby a photocurrent generated between the electrodes 16, 17 through the semiconductor layer 14 is obtained as a readout signal through wirings (not shown).
A reading resolving power of 4-8 lines/mm can be obtained when the distance between the original document P and the sensor 8 is maintained in the order of 0.1 mm, and the distance has to be strictly controlled in order to maintain the above-mentioned resolving power. Accurate controlling the distance is achieved by forming, on the sensor 8, a protective layer 20 antiabrasive to the contact with the original document P.
The protective layer 20 is required to satisfy at least following three functions. A first function is to maintain a constant distance between the sensor element 8 and the original document P, i.e. a spacer function for maintaining an exact spacing between the light-receiving face of the window 18 and the original document P.
A second function is to prevent the deterioration of the sensor element 8 by abrasion, when the original document P is contacted therewith.
A third function is to secure the stability of the sensor element 8 against changes in ambient conditions such as temperature and humidity.
Conventionally, the protective layer 20 has been formed by adhering a translucent material, such as glass, to the substrate 11 so as to cover the sensor element 8, and said adhesion has been obtained by applying an adhesive material to the unillustrated peripheral or end portions of the device. However, such structure in which the glass protective layer and the substrate are both relatively rigid, there is encountered a drawback of a bending stress caused by temperature change, due to the difference in the thermal expansion coefficient of the protective layer and the substrate, or a stress resulting from uneven adhesion, giving eventually rise to mechanical destruction.
Also, such a structure results in an air gap between the protective layer 20 and the light-receiving window 18, leading to a deterioration in image quality and an insufficient ambient stability of the light-receiving face defined by the window 18. In order to avoid such drawback, the adhesive material may be applied on the upper surface of the substrate 11 or of the light-receiving window 18, but the adhesive material generally has an insufficient purity and is unable to ensure a sufficient light transmission, thus deteriorating the reading characteristic of the light-receiving window.
The protective layer 20 may also be formed of a photocurable resin, but there will be encountered drawbacks as first mentioned due to the contraction at resin curing, and the difference in thermal expansion coefficient. Also such resin deteriorates the reading characteristic of the light-receiving face because of the low degree of purificaiton of such resin.